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  <div class="section" id="not-using-items-to-iterate-over-a-dictionary">
<h1>Not using <code class="docutils literal notranslate"><span class="pre">items()</span></code> to iterate over a dictionary<a class="headerlink" href="#not-using-items-to-iterate-over-a-dictionary" title="Permalink to this headline">¶</a></h1>
<p><a class="reference external" href="http://legacy.python.org/dev/peps/pep-0020/">PEP 20</a> states “There should be one– and preferably only one –obvious way to do it.” The preferred way to iterate over the key-value pairs of a dictionary is to declare two variables in a for loop, and then call <code class="docutils literal notranslate"><span class="pre">dictionary.items()</span></code>, where <code class="docutils literal notranslate"><span class="pre">dictionary</span></code> is the name of your variable representing a dictionary. For each loop iteration, Python will automatically assign the first variable as the key and the second variable as the value for that key.</p>
<div class="section" id="anti-pattern">
<h2>Anti-pattern<a class="headerlink" href="#anti-pattern" title="Permalink to this headline">¶</a></h2>
<p>The code below defines a for loop that iterates over a dictionary named <code class="docutils literal notranslate"><span class="pre">d</span></code>. For each loop iteration Python automatically assigns the value of <code class="docutils literal notranslate"><span class="pre">key</span></code> to the name of the next key in the dictionary. Inside of the <code class="docutils literal notranslate"><span class="pre">for</span></code> loop the code uses <code class="docutils literal notranslate"><span class="pre">key</span></code> to access the value of each key of the dictionary. This is a common way for iterating over a dictionary, but it is not the preferred way in Python.</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="n">d</span> <span class="o">=</span> <span class="p">{</span><span class="s2">&quot;first_name&quot;</span><span class="p">:</span> <span class="s2">&quot;Alfred&quot;</span><span class="p">,</span> <span class="s2">&quot;last_name&quot;</span><span class="p">:</span><span class="s2">&quot;Hitchcock&quot;</span><span class="p">}</span>

<span class="k">for</span> <span class="n">key</span> <span class="ow">in</span> <span class="n">d</span><span class="p">:</span>
    <span class="nb">print</span><span class="p">(</span><span class="s2">&quot;</span><span class="si">{}</span><span class="s2"> = </span><span class="si">{}</span><span class="s2">&quot;</span><span class="o">.</span><span class="n">format</span><span class="p">(</span><span class="n">key</span><span class="p">,</span> <span class="n">d</span><span class="p">[</span><span class="n">key</span><span class="p">]))</span>
</pre></div>
</div>
</div>
<div class="section" id="best-practice">
<h2>Best-practice<a class="headerlink" href="#best-practice" title="Permalink to this headline">¶</a></h2>
<div class="section" id="use-items-to-iterate-across-dictionary">
<h3>Use <code class="docutils literal notranslate"><span class="pre">items()</span></code> to iterate across dictionary<a class="headerlink" href="#use-items-to-iterate-across-dictionary" title="Permalink to this headline">¶</a></h3>
<p>The updated code below demonstrates the Pythonic style for iterating through a dictionary. When you define two variables in a <code class="docutils literal notranslate"><span class="pre">for</span></code> loop in conjunction with a call to <code class="docutils literal notranslate"><span class="pre">items()</span></code> on a dictionary, Python automatically assigns the first variable as the name of a key in that dictionary, and the second variable as the corresponding value for that key.</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="n">d</span> <span class="o">=</span> <span class="p">{</span><span class="s2">&quot;first_name&quot;</span><span class="p">:</span> <span class="s2">&quot;Alfred&quot;</span><span class="p">,</span> <span class="s2">&quot;last_name&quot;</span><span class="p">:</span><span class="s2">&quot;Hitchcock&quot;</span><span class="p">}</span>

<span class="k">for</span> <span class="n">key</span><span class="p">,</span><span class="n">val</span> <span class="ow">in</span> <span class="n">d</span><span class="o">.</span><span class="n">items</span><span class="p">():</span>
    <span class="nb">print</span><span class="p">(</span><span class="s2">&quot;</span><span class="si">{}</span><span class="s2"> = </span><span class="si">{}</span><span class="s2">&quot;</span><span class="o">.</span><span class="n">format</span><span class="p">(</span><span class="n">key</span><span class="p">,</span> <span class="n">val</span><span class="p">))</span>
</pre></div>
</div>
</div>
</div>
<div class="section" id="difference-python-2-and-python-3">
<h2>Difference Python 2 and Python 3<a class="headerlink" href="#difference-python-2-and-python-3" title="Permalink to this headline">¶</a></h2>
<p>In python 2.x the above examples using <code class="docutils literal notranslate"><span class="pre">items</span></code> would return a list with tuples containing the copied key-value pairs of the dictionary. In order to not copy and with that load the whole dictionary’s keys and values inside a list to the memory you should prefer the <code class="docutils literal notranslate"><span class="pre">iteritems</span></code> method which simply returns an iterator instead of a list.
In Python 3.x the <code class="docutils literal notranslate"><span class="pre">iteritems</span></code> is removed and the <code class="docutils literal notranslate"><span class="pre">items</span></code> method returns view objects. The benefit of these view objects compared to the tuples containing copies is that every change made to the dictionary is reflected in the view objects.</p>
</div>
<div class="section" id="references">
<h2>References<a class="headerlink" href="#references" title="Permalink to this headline">¶</a></h2>
<ul class="simple">
<li><p><a class="reference external" href="http://legacy.python.org/dev/peps/pep-0020/">PEP 20 - The Zen of Python</a></p></li>
<li><p><a class="reference external" href="https://docs.python.org/2/library/stdtypes.html#dict.iteritems">Python 2 dict.iteritems</a></p></li>
<li><p><a class="reference external" href="https://docs.python.org/3.3/library/stdtypes.html#dict-views">Python 3 dict.items</a></p></li>
</ul>
</div>
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